perf: disable slang support

perf: disable slang support

libs/slang2 is getting picked up from the packages feed, if the slang2
is compiled first and then if you try to compile perf, it fails
due to unmet dependency.

Fixes:
Package perf is missing dependencies for the following libraries:
libslang.so.2

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/19123
(cherry picked from commit 764576eb5b6bae6c7142bd9e560f8c68da177b0d)
Link: https://github.com/openwrt/openwrt/pull/19166
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>